The present invention relates to an improved method of forming helical corrugations in the walls of a cylindrical tube.
U.S. Pat. No. 3,015,355 issued to A. H. Humphrey discloses a method for forming spirally ribbed or helically corrugated tubing in which method a straight walled cylindrical tube, without corrugations, is deformed by radially offsetting the tube wall in selected spots to change the torsional resistance, and then twisting the tube in a turning or twisting machine such as a lathe having a rotatable headstock and a non-rotatable tailstock movable axially toward and away from the headstock. When the tube is twisted in the machine and the axial length of the tube is controlled at the same time, the radial deformations in the tube wall grow axially along a helical path in the tube wall in accordance with the axial and rotational motions of the head and tailstocks.
As described in the referenced patent, the method for forming helical corrugations requires that the radial deformations be generated in the tube wall before the tube is placed in the twisting machine. The operation of generating such deformations necessitates special preparation of the tube prior to insertion in the twisting machine and represents an additional, time-consuming step in the manufacturing of such corrugated tubing. The formation of dimples or other radial deformations must be carried out in a work area distinct from that of the twisting machine and then the tubing with the deformations must be transferred from that work area to the machine for completion of the corrugation-forming process. In addition to a separate work area for forming the dimples, specialized tools and fixtures such as shown in U.S. Pat. No. 3,533,267 issued to Bunnell are required to hold the tubing during the dimple-forming steps. In all, the special preparation of the tube prior to installation in the twisting machine comprises a significant portion of the overall manufacturing process even though the dimples represent a relatively insignificant distortion of the tube walls in comparison to the corrugations which are eventually formed.
It is, accordingly, a general object of the present invention to simplify the prior art process of manufacturing helically corrugated tubes by eliminating the special preparatory operation in which dimples or other radial deformations are impressed in the walls of the tube prior to installation in the twisting machine.